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I recently updated my 1.4x extender to the mk3 however the mk3 doesn't always work on my 500mm F4L (mk1). There is a little play/movement between the lens and the extender or camera body if no extender used but all has been working fine, until the mk3 1.4x.

When the camera and extender are moved to the right of the area of 'play' all is fine but when the camera/extender move to the left there is a failed connectivity, often resulting in an error message. The level of play/movement is extremely minor.

When I use the bare lens or back with my old 1.4x mkii this problem does not occur, its only with the mk3 extender.

The new extender was bought at the end of April and has had very little use in 3 months since I got it..

As the bare lens/body and lens/body/old 1.4x extender combo work perfectly fine this would appear to be an issue with the mkiii extender.

Has anyone else experienced any issues such as this or could it be a compatibility issue between older lens and newer extender?

I have not tried my 1.4 Mk3 (+ 2x Mk3) with the 500 Mk1 but it has been used regularly with the 300 F2.8 L IS Mk1, 800 F5.6 L IS and a friend's 600 F4 L IS Mk1 - no contact issues. Do you have access to any other lenses that will accept this extender? Is it OK on them?

Other than dirty contacts (unlikely on a new extender) it should work perfectly. However you mentioned "play" with the extender in place - are all the mounts tightened down properly? You do get more play when using extenders as there are 2 extra mounts in the chain so if there is a little looseness somewhere it might cause this issue. Remember when you are checking the screws on the mounts that they are JIS standard not Phillips so go carefully.

I think that a loose mount is unlikely but it has happened to others so it is worth a look.

The mounts are fine and the mk2 1.4 works fine with the 500mm. The play is the slight movement between lens and body/extender in a rotational movement which has always been there. Various forums indicate that this movement is normal.